Source: Data Sheet, Department of Astronautics, National Air and Space
Museum, Smithsonian Institution.

Explorer-I and Jupiter-C

The First United States Satellite and Space Launch Vehicle

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Explorer-I, officially known as Satellite 1958 Alpha, was the first United
States earth satellite and was sent aloft as part of the United States program
for the International Geophysical Year 1957-1958. It was designed and built by
the Jet Propulsion Laboratory (JPL) of the California Institute of Technology
under the direction of Dr. William H.
Pickering. The satellite instrumentation of Explorer-I was designed and
built by Dr. James Van
Allen of the State University of Iowa.

The satellite was launched from Cape Canaveral (now Cape Kennedy) in Florida
at 10:48 P.M. EST on 31 January 1958 by the Jupiter-C vehicle--a special
modification of the Redstone ballistic missile--that was designed, built, and
launched by the Army Ballistic Missile Agency (ABMA) under the direction of Dr. Wernher Von Braun.
Jupiter-C, a direct descendant of the German A-4 (V-2) rocket, was originally
developed in 1955-1956 as a high-performance rocket for testing purposes.

The Jupiter-C has its origins in the United States Army's Project Orbiter in
1954. The project was canceled in 1955, however when the decision was made to
proceed with Project Vanguard.

Following the launch of the Soviet Sputnik I on 4 October 1957, ABMA was
directed to proceed with the launching of a satellite using the Jupiter-C, which
had already been flight-tested in nose-cone re-entry tests for the Jupiter
intermediate-range ballistic missile (IRBM). Working closely together, ABMA and
JPL completed the job of modifying the Jupiter-C and building the Explorer-I in
84 days.

Once in orbit, the cosmic ray equipment of Explorer-I indicated a much lower
cosmic ray count than had been anticipated. Dr. Van Allen theorized that the
equipment may have been saturated by very strong caused by the existence of a
belt of charged particles trapped in space by the earth's magnetic field. The
existence of these Van Allen Belts, discovered by Explorer-I, was confirmed by
Explorer-III, which was launched by a Jupiter-C on 26 March 1958.

The discovery of the Van Allen Belts by the Explorer satellites was
considered to be one of the outstanding discoveries of the International
Geophysical Year.

EXPLORER-I

Characteristics

Explorer-I was placed in an orbit with a perigee of 224 miles and an apogee
of 1,575 miles having a period of 114.9 minutes. Its total weight was 30.66
pounds, of which 18.35 pounds were instrumentation. The instrument section at
the front end of the satellite and the empty scaled-down Sergeant fourth-stage
rocket casing orbited as a single unit, spinning around its long axis at 750
revolutions per minute.

Instrumentation consisted of a cosmic-ray detection package, an internal
temperature sensor, three external temperature sensors, a nose-cone temperature
sensor, a micrometeorite impact microphone, and a ring of micrometeorite erosion
guages. Data from these instruments were transmitted to the ground by a
60-milliwatt transmitter operating on 108.03 megacycles and a 10-milliwatt
transmitter operating on 108.00 megacycles.

Transmitting antennas consisted of two fibre-glass slot antennas in the body
of the satellite itself and four flexible whips forming a turnstile antenna. The
rotation of the satellite about its long axis kept the flexible whips extended.

The external skin of the instrument section was painted in alternate strips
of white and dark green to provide passive temperature control of the satellite.
The proportions of the light and dark strips were determined by studies of
shadow-sun-light intervals based on firing time, trajectory, orbit, and
inclination.

Electrical power was provided by nickel-cadmium chemical batteries [sic]* that made
up approximately 40 percent of the payload weight. These provided power that
operated the high power transmitter for 31 days and the low-power transmitter
for 105 days.

Because of the limited space available and the requirements for low weight,
the Explorer-I instrumentation was designed and built with simplicity and high
reliability in mind. It was completely successful.

Jupiter-C

Characteristics

The Jupiter-C rocket was originally developed to test the ablative
re-entry nose cone of the Jupiter IRBM, although its satellite-launching
capabilities were recognized at the time it was designed.

The vehicle consists of a modified Redstone ballistic missile topped by
three solid-propellant upper stages. The tankage of the Redstone was
lengthened by eight feet to provide additional propellant. The instrument
compartment is also smaller and lighter than the Redstone's. The second
and third stages are clustered in a "tub" atop the vehicle, while the
fourth stage is atop the tub itself. The second stage is an outer ring of
eleven scaled-down Sergeant rocket engines; the third stage is a cluster
of three scaled down Sergeant rockets grouped within. These are held in
position by bulkheads and rings and are surrounded by a cylindrical outer
shell. The webbed base plate of the shell rests on a ball-bearing shaft
mounted on the first-stage instrument section. Two electric motors spin in
the tub at a rate varying from 450 to 750 rpm to compensate for thrust
imbalance when the clustered motors fire. The rate of spin is varied by a
programmer so that it does not couple with the changing resonant frequency
of the first stage during flight.

The upper-stage tub was spun-up before launch. During first-stage
flight, the vehicle was guided by a gyro-controleld autopilot controlling
both air-vanes and jet vanes on the first stage by means of servos.
Following a vertical launch from a simple steel table, the vehicle was
programmed so that it was travelling at an angle of 40 degrees from the
horizontal at burnout of the first stage, which occurred 157 seconds after
launch. At first-stage burnout, explosive bolts fired and springs
separated the instrument section from the first-stage tankage. The
instrument section and the spinning tub were slowly tipped to a horizontal
position by means of four air jets located at the base of the instrument
section. When the apex of the vertical flight occurred after a coasting
flight of about 247 seconds, a radio signal from the ground ignited the
eleven-rocket cluster of the second stage, separating the tub from the
instrument section. The third and fourth stages were fired in turn to
boost the satellite and fourth stage to an orbital velocity of 18,000
miles per hour.

When used as a satellite launching vehicle, the Jupiter-C is sometimes
referred to as the Juno-I.

*These were not nickel cadmium batteries but were Mallory type RM mercury cells.
See: Ludwig, George H. "Cosmic- Ray Instrumentation in the First U.S. Earth Satellite" The Review of Scientific Instruments, Volume 30, Number 4, April 1959.
See also: Space Activities Summary, S-58-1, prepared by Office of Public Information, U.S. National Aeronautics and Space Administration, December 1960.